Offset memory component automatic calibration (autocal) error recovery for a memory sub-system

ABSTRACT

Several embodiments of memory devices and systems with offset memory component automatic calibration error recovery are disclosed herein. In one embodiment, a system includes at least one memory region and calibration circuitry. The memory region has memory cells that read out data states in response to application of a current read level signal. The calibration circuitry is operably coupled to the at least one memory region and is configured to determine a read level offset value corresponding to one or more of a plurality of offset read level test signals, including a base offset read level test signal. The base offset read level test signal is offset from the current read level signal by a predetermined value. The calibration circuitry is further configured to output the determined read level offset value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/690,546, filed Nov. 21, 2019; which is a continuation of U.S. application Ser. No. 16/102,092, filed Aug. 13, 2018, now U.S. Pat. No. 10,529,433; each of which is incorporated herein by reference in its entirety.

This application contains subject matter related to an U.S. Patent Application by Bruce A. Liikanen and Gerald L. Cadloni titled “AUTOMATIC CALIBRATION (AUTOCAL) ERROR RECOVERY FOR A MEMORY SUB-SYSTEM.” The related application is assigned to Micron Technology, Inc., and is identified as U.S. application Ser. No. 16/102,067, filed Aug. 13, 2018, now U.S. Pat. No. 10,446,241.

TECHNICAL FIELD

The disclosed embodiments relate to memory sub-systems, and, in particular, to memory sub-systems with offset memory component automatic calibration error recovery.

BACKGROUND

Memory devices can employ flash media to persistently store large amounts of data for a host device, such as a mobile device, a personal computer, or a server. Flash media includes “NOR flash” and “NAND flash” media. NAND-based media is typically favored for bulk data storage because it has a higher storage capacity, lower cost, and faster write speed than NOR media. The memory cells in NAND flash employ a charge storage structure, (e.g., a floating gate structure or a charge trapping structure) for storing charge to represent different data states. The cells are programmed by transferring electrons through a thin dielectric layer (e.g., a tunnel oxide) from a channel to, e.g., a floating gate or a charge trapping layer within the charge storage structure. The amount of charge stored in a memory cell represents one or more threshold voltages that are indicative of the voltage(s) required to form a conductive path within the channel, (e.g., depending on the amount of electrons stored on the floating gate or the charge trapping layer).

One drawback of flash memory and other non-volatile memory is that the threshold voltages of the individual memory cells can change as, over time, the memory device erases and writes data to the memory. For example, over multiple erase and write cycles, electrons can become trapped within the tunnel oxide of a memory cell, causing the threshold voltage(s) of the cell to gradually increase. This phenomenon, if uncorrected, can result in bit errors during a read of the data stored in the memory cell.

In some circumstances, error correcting code (ECC) techniques may be employed to detect and correct bit errors if the number of bit errors does not exceed the correction capacity of the code. Eventually, however, as more electrons are trapped within the tunnel oxide layers of more and more memory cells in a memory device, the number of memory cells with unreadable data states (e.g., due to shifted threshold voltages) may exceed the correction capacity of the ECC. When this happens, the memory controller is no longer able to efficiently or properly read out data from the affected memory regions.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the disclosure. The drawings, however, should not be taken to limit the disclosure to the specific embodiments, but are for explanation and understanding only

FIG. 1 is a block diagram of a system having a memory device configured in accordance with an embodiment of the present technology.

FIG. 2 is a message flow diagram illustrating various data exchanges between components of a memory device in accordance with an embodiment of the present technology.

FIGS. 3A and 3B are plots of channel current versus applied voltage of a memory cell within a memory region of a memory device in accordance with an embodiment of the present technology.

FIGS. 4A-G are various plots, tables, and diagrams illustrating offset read level calibration of a memory region in accordance with embodiments of the present technology.

FIGS. 5A and 5B are flow diagrams illustrating methods for operating a memory device in accordance with an embodiment of the present technology.

FIG. 6 is a schematic view of a system that includes a memory device in accordance with embodiments of the present technology.

DETAILED DESCRIPTION

As described in greater detail below, the present technology relates to memory devices and related systems with offset read level calibration. A person skilled in the art, however, will understand that the technology may have additional embodiments and that the technology may be practiced without several of the details of the embodiments described below with reference to FIGS. 1-6. In the illustrated embodiments below, the memory devices are described in the context of devices incorporating NAND-based storage media (e.g., NAND flash). Memory devices configured in accordance with other embodiments of the present technology, however, can include other types of memory devices and/or can include main memories that are not NAND-based (e.g., NOR-based) or only partially NAND-based.

One embodiment of the present technology is a system comprising a memory region having a plurality of memory cells configured to read out data states in response to application of a current read level signal. The system further comprises calibration circuitry operably coupled to the memory region. The calibration circuitry is configured to (1) measure, for a portion of the memory region, a performance characteristic for each of a plurality of offset read level test signals, wherein a base offset read level test signal of the plurality of offset read level test signals is offset from the current read level signal by a predetermined value; (2) determine a read level offset value corresponding to one or more of the plurality of offset read level test signals based on the performance characteristics; and (3) output the read level offset value.

A read level offset value may be used to update a corresponding current read level signal for a portion of the memory region. In this manner, the current read level signals for the memory component of the memory device can be calibrated to account for shifts in the threshold voltages in the memory cells of the memory component, which, in turn, decreases the occurrence of bit errors and increases the life of the memory device. In some instances, shifts in threshold voltages may be relatively large, such that utilizing read level test signals at, around, or near a current read level signal may not yield a better or optimal read level offset value. Accordingly, as described herein, a base offset read level test signal can be offset from the current read level signal. This can effectively increase the range of read level test voltages that a memory system can use for calibration. In addition, an amount to which the base offset read level test signal is offset from the current read level signal may be determined by a system to further optimize calibration as described herein.

FIG. 1 is a block diagram of a system 101 having a memory sub-system (also hereinafter referred to as a “memory device” (e.g., memory device 100)) configured in accordance with an embodiment of the present technology. An example of a memory sub-system is a storage system, such as a solid-state drive (SSD). In some embodiments, the memory sub-system is a hybrid memory/storage sub-system. In general, a host system can utilize a memory sub-system that includes one or more memory components. The host system can provide data to be stored at the memory sub-system and can request data to be retrieved from the memory sub-system. As shown, the memory device 100 includes one or more memory components (e.g., memory component 102 (e.g., NAND flash)) and a controller 106 operably coupling the memory component 102 to a host device 108 (e.g., an upstream central processor (CPU)). The memory component 102 includes a plurality of memory regions, or memory units 120, which each include a plurality of memory cells 122. Memory units 120 can be individual memory dies, memory planes in a single memory die, a stack of memory dies vertically connected with through-silicon vias (TSVs), or the like. In one embodiment, each of the memory units 120 can be formed from a semiconductor die and arranged with other memory unit dies in a single device package (not shown). In other embodiments, one or more of the memory units 120 can be co-located on a single die and/or distributed across multiple device packages. The memory cells 122 can include, for example, floating gate, charge trap, phase change, ferroelectric, magnetoresitive, and/or other suitable storage elements configured to store data persistently or semi-persistently. The memory component 102 and/or the individual memory units 120 can also include other circuit components (not shown), such as multiplexers, decoders, buffers, read/write drivers, address registers, data out/data in registers, etc., for accessing and/or programming (e.g., writing) the memory cells 122 and other functionality, such as for processing information and/or communicating with the controller 106.

Memory cells 122 can be arranged in rows 124 (e.g., each corresponding to a word line) and columns 126 (e.g., each corresponding to a bit line). Furthermore, adjacent word lines 124 can be arranged into one or more word line groups that compose a memory block 128. Each word line 124 can span one or more memory pages, depending upon the number of data states the memory cells 122 of that word line 124 are configured to store. For example, a single word line 124 of memory cells 122 in which each memory cell 122 stores one of two data states (e.g., SLC memory cells configured to store one bit each) can span a single memory page. Alternatively, a single word line 124 of memory cells 122 in which each memory cell 122 stores one of four data states (e.g., MLC memory cells configured to store two bits each) can span two memory pages. Moreover, memory pages can be interleaved so that a word line 124 comprised of memory cells 122 configured to store one of two data states in each cell (e.g., SLC memory cells) can span two memory pages, in an “even-odd bit line architecture,” where all the memory cells 122 in odd-numbered columns 126 of a single word line 124 are grouped as a first memory page, and all the memory cells 122 in even-numbered columns 126 of the same word line 124 are grouped as a second memory page. When even-odd bit line architecture is utilized in a word line 124 of memory cells 122 that store larger numbers of data states in each cell (e.g., memory cells configured as MLC, TLC, QLC, etc.), the number of memory pages per word line 124 can be even higher (e.g., 4, 6, 8, etc.).

Each column 126 can include a string of series-coupled memory cells 122 coupled to a common source. The memory cells 122 of each string can be connected in series between a source select transistor (e.g., a field-effect transistor) and a drain select transistor (e.g., a field-effect transistor). Source select transistors can be commonly coupled to a source select line, and drain select transistors can be commonly coupled to a drain select line.

In other embodiments, the memory cells 122 can be arranged in different types of groups and/or hierarchies than those shown in the illustrated embodiments. Further, while shown in the illustrated embodiments with a certain number of memory cells, rows, columns, blocks, and memory units for purposes of illustration, in other embodiments, the number of memory cells, rows, columns, blocks, and memory units can vary, and can be larger or smaller in scale than shown in the illustrated examples. For example, in some embodiments, the memory device 100 can include only one memory unit 120. Alternatively, memory device 100 can include two, three, four, eight, ten, or more (e.g., 16, 12, 64, or more) memory units 120. While the memory units 120 are shown in FIG. 1 as including two memory blocks 128 each, in other embodiments, each memory unit 120 can include one, three, four eight, or more (e.g., 16, 32, 64, 100, 128, 256 or more memory blocks). In some embodiments, each memory block 128 can include, e.g., 2¹⁵ memory pages, and each memory page within a block can include, e.g., 2¹² memory cells 122 (e.g., a “4k” page).

The memory component 102 further includes a calibration component, or calibration circuitry 110 (shown schematically), operably coupled to at least one of the memory units 120. In some embodiments, the calibration circuitry 110 can be located on the same memory die as an individual memory unit 120. In these and other embodiments, the calibration circuitry 110 may be dedicated to a corresponding memory unit 120 or multiple memory units, including memory units on different die. The calibration circuitry 110 can include circuit components, such as multiplexers, decoders, buffers, read/write drivers, address registers, data out/data in registers, etc. In some embodiments, the calibration circuitry 110 can be circuitry separate from other on-chip circuitry used for accessing and/or programming (e.g., reading and/or writing) the memory cells 122 and/or for providing other functionality, such as for processing information and/or communication with the controller 106. The calibration circuitry 110 is configured to implement the offset NAND autocal devices, systems, methods, and computer readable medium with executable instructions as described herein.

The calibration circuitry 110 can be separate from the memory component 102, or can be incorporated within the controller 106, the host 108, or another device. Various aspects of the calibration circuitry 110 may also exist on different devices including the memory component 102, the controller 106, the host 108, or another device. Accordingly, the methods and functionalities discussed herein associated with the calibration circuitry 110 may be performed in whole or in part by or at any combination of the memory component 102, the controller 106, the host 108, or another device. Other devices may be connected to the memory component 102, the controller 106, and/or the host 108 through a network, for example. Other devices may be other memory devices, a computing device configured to manage and communicate with memory devices, or another type of computing device.

The controller 106 can be a microcontroller, special purpose logic circuitry (e.g., a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), or other suitable processor. The controller 106 can include a processor 130 configured to execute instructions stored in memory. In the illustrated example, the memory of the controller 106 includes an embedded memory 132 configured to perform various processes, logic flows, and routines for controlling operation of the memory device 100, including managing the memory component 102 and handling communications between the memory device 100 and the host device 108. In some embodiments, the embedded memory 132 can include memory registers storing, e.g., memory pointers, fetched data, etc. The embedded memory 132 can also include read-only memory (ROM) for storing micro-code. While the exemplary memory device 100 illustrated in FIG. 1 has been illustrated as including a controller 106, in another embodiment of the present technology, a memory device may not include a controller, and may instead rely upon external control (e.g., provided by an external host, or by a processor or controller separate from the memory device).

In operation, the controller 106 can directly write or otherwise program (e.g., erase) the various memory regions of the memory component 102, such as by writing to groups of memory pages and/or memory blocks 128. In NAND-based memory, a write operation often includes programming the memory cells 122 in selected memory pages with specific data values (e.g., a string of data bits having a value of either logic 0 or logic 1). An erase operation is similar to a write operation, except that the erase operation re-programs an entire memory block 128 or multiple memory blocks 128 to the same data state (e.g., logic 0).

The controller 106 communicates with the host device 108 over a host-device interface 115. In some embodiments, the host device 108 and the controller 106 can communicate over a serial interface, such as a serial attached SCSI (SAS), a serial AT attachment (SATA) interface, a peripheral component interconnect express (PCIe), or other suitable interface (e.g., a parallel interface). The host device 108 can send various requests (in the form of, e.g., a packet or stream of packets) to the controller 106. A request can include a command to write, erase, return information, and/or to perform a particular operation (e.g., a TRIM operation).

The controller 106 can also calibrate signals used to program and read from the memory component 102. In various embodiments described below, the controller 106 can send a calibration signal including to the calibration circuitry 110 to instruct the calibration circuitry 110 to self-calibrate one or more memory regions of the memory component 102, which can improve or optimize the signaling (e.g., voltage signaling) used to read the data state of the individual memory cells 122, such as a selected group of memory cells 122 (e.g., multiple memory cells associated with a memory page, multiple memory pages associated with a word line, multiple word lines associated with a word line group, etc.). As described herein, that calibration signal can include a predetermined amount at which a base read level test signal should be offset from a current read level signal.

FIG. 2 is a message flow diagram illustrating various data exchanges between the memory component 102 (FIG. 1) and the controller 106 (FIG. 1) of the memory device 100 (FIG. 1) and/or the host device 108 (FIG. 1) in accordance with one embodiment of the present technology. The controller 106 can read from the memory component 102 by sending a read request 230 to the memory component 102. The read request 230 can include physical addresses of one or more memory regions within the memory component 102 and/or read level signals (e.g., read level voltages) of those memory regions. In response to the read request 230, the memory component 102 can apply the read level signals to the corresponding memory regions and read out data states stored on the memory cells 122 (FIG. 1) within the memory regions. The memory component 102 can return the data states of the memory cells 122 to the controller 106 in a read data return message 231.

The controller 106 can also periodically calibrate the read level signals of one or more select memory regions within the memory component 102 according to the embodiments described herein. The read level signals may be periodically calibrated to account for, e.g., a shift of one or more threshold voltages of memory cells within the memory regions. To calibrate the read level signals of selected memory regions within the memory component 102, the controller 106 can send a calibrate command 232 to the calibration circuitry 110. The calibrate command 232 can include physical addresses of the selected memory regions and/or current read level signals (e.g., default read level signals and/or previously calibrated read level signals) of the selected memory regions. The calibrate command 232 can also include a predetermined amount at which a base read level test signal should be offset from a current read level signal. In accordance with one embodiment of the present technology, the calibration circuitry 110 can calibrate the selected memory regions in response to the calibrate command 232. As part of the calibration, the calibration circuitry 110 can return one or more read level offset values 233 to the controller 106 that can represent calculated offsets from the current read level signals of the selected memory regions. The controller 106 can then use the read level offset values 233 to update the current read level signals to arrive at an improved read level signal for the selected memory regions. The controller 106 and the host device 108 may then continue to program and/or read the memory component 102 using the calibrated read level signals.

As shown in FIG. 2, read level calibration occurs internal to the memory component 102. This can reduce the time required to perform read level calibration because it eliminates the need for multiple messages between the controller 106 and the memory component 102. For example, the controller 106 can send a single calibration signal 232 to the memory component 102. In response, the calibration circuitry 110 of the memory component 102 internally performs read level calibration using an offset base read level test signal (as described in greater detail below) and outputs a read level offset value 233 to controller 106. Thus, the controller 106 does not send multiple read signals (e.g., corresponding to read operations at with different read level voltages) to the memory component 102 to calibrate one or more selected memory regions during read level calibration. Therefore, the execution time of read level calibration can be reduced, freeing up the memory device 100 to perform other requests and/or tasks. In some embodiments according to the present technology, however, the calculations performed using the base offset read level test signal (and any other offset read level test signals) can be performed elsewhere, such as at the controller 106 and/or the host 108. In such embodiments, the outputs of the various offset read level test signals may be sent back to the controller 106 and/or the host 108 in lieu of the read level offset value 233. In this way, the calculations related to calibrating one or more read level offset values can be performed at the controller 106 and/or the host 108. Once the one or more read level offset values is calculated, that value can be stored at the controller 106 and/or the host 108, and may also be sent to the calibration circuitry 110 and/or the memory component 102.

FIGS. 3A and 3B are plots of channel current versus applied voltage of a memory cell, such as one of the memory cells 122 (FIG. 1) within a memory region of the memory component 102 (FIG. 1). Referring to FIG. 3A, the memory cell has a first data state threshold voltage V_(T1) and a second data state threshold voltage V_(T2) representing the voltages required to create a conductive path in the channel of the memory cell programmed with a first data state (e.g., “0”) and a second data state (e.g., “1”), respectively. The memory region containing the memory cell has a current read level signal V_(a) (e.g., a current read level voltage) between the first data state threshold voltage V_(T1) and the second data state threshold voltage V_(T2). As the current read level signal V_(a) is applied to the memory region, the data state stored on the memory cell may be determined (e.g., read) from the memory cell. For example, if the current read level signal V_(a) is applied to the memory region and current read from the memory cell in response to the current read level signal V_(a) is not negligible (e.g., not zero and/or above a threshold value), the data state stored on the memory cell is determined to be in the first data state (e.g., “0”). If, however, the current read from the memory cell is negligible (e.g., zero and/or below a threshold value), the data state stored on the memory cell illustrated in FIG. 3A is determined to be in the second data state (e.g., “1”).

In some memory devices, read level signals of a memory region are initially programmed by the manufacturer of a memory device (e.g., at the time of manufacture or initial configuration) and may thereafter remain unchanged for the life of the memory device. However, as the memory region is repeatedly programmed and/or erased, or the memory region is subject to various operations (e.g., writing adjacent memory regions, reading adjacent memory regions, etc.) and/or environmental conditions (e.g., temperatures), the threshold voltages of the memory cells within the memory region can change (e.g., due to the trapping of electrons in the tunnel oxides thereof, leakage of charge from the charge trapping structure, etc.). FIG. 3B illustrates the effect of this phenomenon on the threshold voltages of the memory cell in accordance with one embodiment of the present technology. In the embodiment illustrated in FIG. 3B, the first data state threshold voltage V_(T1) and the second data state voltage V_(T2) have increased in relation to the current read level signal V_(a) of the memory region containing the memory cell. As shown, applying the current read level signal V_(a) will result in a negligible current read from the memory cell regardless of whether the memory cell has been programmed with the first data state or the second data state. In other words, a shift in the threshold voltages of the memory cell in relation to the current read level signal of the memory region results in an increased possibility of a bit error from that memory cell. As the occurrence of bit errors begins to gradually increase across multiple memory cells within the memory region, error code correction (ECC) techniques may eventually become ineffective, at which point the controller 106 (FIG. 1) may be unable to properly read out the data stored in the memory region. Thus, calibrating the read level signals of the memory regions within a memory device can significantly increase the performance and/or useful life of the memory device.

Although the memory cell in FIGS. 3A-B is illustrated with two threshold voltages, memory cells may have a different number of threshold voltages (e.g., four, eight, sixteen, etc. threshold voltages) representing more than two data states (e.g., four, eight, sixteen, etc. data states). Similarly, memory regions containing these memory cells may have more than one read level signal (e.g., three, seven, fifteen, etc. read level signals). Memory cells having a larger number of threshold voltages (e.g., MLC memory cells, TLC memory cells, QLC memory cells, etc.) tend to be more prone to bit errors because the tolerances between data states within the memory cells are smaller. Thus, even a small shift in the threshold voltages of these memory cells can begin causing bit errors within fewer write and erase cycles than in memory cells having fewer threshold voltages (e.g., SLC memory cells). Therefore, memory regions containing memory cells having a larger number of threshold voltages can experience a greater benefit from offset read level signal calibration.

FIGS. 4A-G are various plots, tables, and diagrams illustrating read level calibration of a memory region in accordance with embodiments of the present technology. After the memory component 102 (FIG. 1) receives a calibration command 232 (FIG. 2) from the controller 106 (FIG. 1) that includes, for example, the physical address(es) of the memory region, the calibration circuitry 110 (FIG. 1) of the memory component 102 can apply various test signals (e.g., test voltages) to the memory region to calibrate one or more of the current read level signals (e.g., the current read level voltages) of the memory region. The current read level signals can be the default read level signals (e.g., the default read level voltages) initially programmed by the manufacturer and/or the current read level signals can be previously calibrated read level signals. In FIG. 4A, read level calibration of a current read level signal V_(A) (e.g., current read level signal V_(a); FIGS. 3A-B) of a memory region without using offset read level test signals is shown. In the embodiments illustrated in FIGS. 4B-G, read level calibration of a current read level signal V_(A) (e.g., current read level signal V_(a); FIGS. 3A-B) of a memory region using offset read level test signals is shown.

As shown in plot 440 of FIG. 4A, the calibration circuitry 110 applies five test signals to the memory region containing the depicted memory cell. More specifically, the calibration circuitry 110 applies the current read level signal V_(A) (i.e., a base read level test signal V_(A)), two upper test signals V_(U1), V_(U2) offset (e.g., at 20 mV intervals) above the base read level test signal V_(A), and two lower test signals V_(L1), V_(L2) offset (e.g., at 20 mV intervals) below the base read level test signal V_(A). In various embodiments, the calibration circuitry 110 can apply a different number of test signals (e.g., three, seven, nine, etc. test signals) to the memory region and/or can apply a different arrangement of test signals (e.g., three upper test signals and one lower test signal or vice versa) to the memory region. In various embodiments, the test signals may be uniformly spaced apart and/or the spacing between the test signals may vary.

According to an embodiment of the present technology, the base read level test signal applied by the calibration circuitry 110 can be offset from the current read level signal V_(A). For example, as shown in plot 442 of FIG. 4B, a base offset read level test signal V_(B) is offset from the current read level signal V_(A) by a predetermined voltage offset 443 (e.g., specified by the calibration command received from the controller 106). In various embodiments, the predetermined voltage offset 443 may be any value, and may be positive or negative such that the base offset read level test signal V_(B) has a higher or lower voltage than the current read level signal V_(A). The base offset read level test signal V_(B) can be used as the base test signal for a plurality of offset read level test signals, as described below.

As shown in plot 444 of FIG. 4C, the calibration circuitry 110 can utilize the predetermined voltage offset 443 to determine the base offset read level test signal V_(B) and two offset upper test signals V_(U1), V_(U2) offset (e.g., at 20 mV intervals) above the offset base read level test signal V_(B), and two offset lower test signals V_(L1), V_(L2) offset (e.g., at 20 mV intervals) below the offset base read level test signal V_(B). In various embodiments, the calibration circuitry 110 can apply a different number of test signals (e.g., three, seven, nine, etc. test signals) to the memory region and/or can apply a different arrangement of test signals (e.g., three upper test signals and one lower test signal or vice versa) to the memory region. In various embodiments, the test signals may be uniformly spaced apart and/or the spacing between the test signals may vary. Accordingly, as shown in FIG. 4C, the calibration circuitry can apply a plurality of offset read level test signals including a base offset read level test signal V_(B) that is offset from the current read level signal V_(A). Such a functionality can be advantageous in certain circumstances because read level test signals near the current read level signal V_(A) may not include a test signal at or near the first data state threshold voltage V_(T1), potentially making a calibration process ineffective.

In various embodiments, the calibration circuitry may perform a first calibration using a base read level test signal the is not offset from the current read level signal (e.g., where the voltage offset 443 specified in the calibration command is 0), and may perform a second calibration using a base offset read level test signal according to the present technology. In this way, the calibration circuitry 110 can effectively perform calibrations over a wider range of read level test voltage values. In some embodiments, if one of the calibrations is ineffective, the calibration circuitry 110 may not change a current read level signal (i.e., a read level offset value output by the calibration circuitry 100 may be zero).

In various embodiments, the calibration circuitry may also perform different calibrations using different predetermined offset levels 443. For example, in a first calibration, the calibration circuitry 110 may offset the base read level test signal up by 50 mV. In a second calibration, the calibration circuitry 110 may offset the base read level test signal down by 50 mV (i.e., the predetermined voltage offset 443 is negative). In a third calibration, the calibration circuitry 110 may offset the base read level test signal up by 100 mV. In a fourth calibration, the calibration circuitry 110 may offset the base read level test signal down by 100 mV (i.e., the predetermined voltage offset 443 is negative). In this way, the system may perform calibrations at varying offset levels to better determine accurate current read level signals for a particular data state of a memory region.

FIGS. 4D-4G show an embodiment for determining a read level offset value as described below based on the application of a plurality of offset read level test signals, including a base offset read level test signal offset from a current read level signal as described herein. Referring to FIGS. 4D and 4E, the calibration circuitry 110 applies each of the offset test signals to the memory region (e.g., the offset test signals shown in FIG. 4C), and the calibration circuitry 110 can determine a number of memory cells within the memory region (e.g., a count) that output a specified data state (e.g., a current above a threshold value and/or a current below a threshold value). In the embodiment illustrated in FIG. 4D, the calibration circuitry 110 determines a count as each of the offset test signals V_(L2), V_(L1), V_(B), V_(U1), V_(U2) are applied to the memory region. As shown in table 450 of FIG. 4D,1 when the calibration circuitry 110 applies the lower test signal V_(L2) to the memory region, 586 memory cells output the specified data state. Similarly, when the calibration circuitry 110 applies the lower test signal V_(L1), 534 memory cells output the specified data state; when the calibration circuitry 110 applies the base offset test signal V_(B), 493 memory cells output the specified data state; when the calibration circuitry 110 applies the upper test signal V_(U1), 467 memory cells output the specified data state; and when the calibration circuitry 110 applies the upper test signal V_(U2) to the memory region, 411 memory cells output the specified data state.

After the calibration circuitry 110 obtains counts indicative of the number of memory cells that output the specified data state as each offset read level test signal is applied to the memory region, the calibration circuitry 110 can calculate count differences between the counts corresponding to adjacent test signals. For example and as shown in table 450 of FIG. 4D, the calibration circuitry 110 calculates four count differences cd₁-cd₄ as follows: (1) the count difference between the lower test signal V_(L2) and the lower test signal V_(L1); (2) the count difference between the lower test signal V_(L1) and the base offset test signal V_(B); (3) the count difference between the base offset test signal V_(B) and the upper test signal V_(U1); and (4) the count difference between the upper test signal V_(U1) and the upper test signal V_(U2). The calibration circuitry 110 can then compare the count differences to determine a minimum count difference. As shown in table 450 of FIG. 4D and histogram 460 of FIG. 4E, the count difference cd₃ (i.e., the difference in counts obtained with the base offset test signal V_(B) and the upper test signal V_(U1)) is determined to be minimum count difference m in the embodiment illustrated in FIGS. 4B-G.

The bit counts and count differences shown in FIGS. 4D and 4E are collectively referred to herein as performance characteristics. The relative differences and extrapolations between relative differences discussed below with respect to FIGS. 4F and 4G are also performance characteristics of the output of a memory region based on the application of test signals. One, some, or all of these performance characteristics can be utilized to determine one or more read level offset values that are output by the calibration circuitry 110 to a controller and/or host. As described herein, a read level offset value can be utilized to adjust a current read level signal for reading the data states of memory cells.

In some embodiments, the calibration circuitry 110 may determine a read level offset value based on the minimum count difference m shown in FIGS. 4D and 4E. For example, the calibration circuitry 110 may determine the read level offset value to be the read level test value nearest the minimum count difference m that is also closest to the base offset test signal V_(B), which in this instance would be the base offset test signal V_(B) voltage level. In another example, the read level offset value may be determined to be the read level test value nearest the minimum count difference m that is farther from the base offset test signal V_(B), which in this instance would be the upper test signal V_(U1) voltage level. In another example, the calibration circuitry may determine the read level offset value to be somewhere between the two test values between which the minimum count difference m was found. For example, the read level offset value may be determined to be a voltage level halfway between the base offset test signal V_(B) and the upper test signal V_(U1). In various embodiments, other methods for determining the read level offset value may be utilized. For example, a test signal having the highest bit count as shown in FIG. 4D may be determined to be the read level offset value. Another embodiment for determining a read level offset value is described below with respect to FIGS. 4F and 4G.

In various embodiments, the read level offset value may be output as described above (e.g., the read level offset value is output as a voltage value offset from the base offset read level test signal V_(B)). In such embodiments, the controller 106 and/or the host 108 may add the read level offset value to the amount the base offset read level test signal V_(B) is offset from the current read level signal V_(A). In this way, the controller 106 and/or the host 108 can determine how far to adjust the current read level signal V_(A) to yield a new, calibrated current read level signal. In other embodiments, this calculation may occur in the calibration circuitry, such that the output read level offset value includes the amount the base offset read level test signal V_(B) was offset from the current read level signal V_(A). In such an embodiment, the controller 106 and/or the host 108 would not have to adjust the outputted read level offset value before adjusting the current read level signal.

Referring now to FIG. 4F, once the calibration circuitry 110 obtains the minimum count difference m, the calibration circuitry 110 can calculate differences between the other count differences and the minimum count difference (i.e., differences relative to the minimum count difference). For example and as shown in table 470 of FIG. 4F, the calibration circuitry 110 can calculate relative differences rd₁-rd₃ between each of the count differences cd₁, cd₂, cd₄ and the minimum count difference m (i.e., the count difference cd₃). As shown in the illustrated embodiment, the relative difference rd₁ between the count difference cd₁ and the minimum count difference m is 26. Similarly, the relative difference rd₂ between the count difference cd₂ and the minimum count difference m is 15, and the relative difference rd₃ between the minimum count difference m and the count difference cd₄ is −29. The relative difference rd₃ is represented as a negative value in the illustrated embodiment because this relative difference corresponds to test signals, counts, and a count difference that are offset above and to the right of the minimum count difference m.

After the calibration circuitry 110 calculates the relative differences between the other count differences and the minimum count difference, the calibration circuitry 110 can extrapolate a value between adjacent relative differences having opposite signs (i.e., between relative differences corresponding to count differences adjacent to and surrounding the minimum count difference). FIG. 4G is a plot 480 graphically representing the extrapolation calculation. As shown, a sign change occurs between the graphical representation of the relative difference rd₂ and the graphical representation of the relative difference rd₃. Thus, the relative differences rd₂ and the relative difference rd₃ are the relative differences corresponding to the count differences adjacent to and surrounding the minimum count difference m. In contrast, no sign change occurs between the graphical representation of the relative difference rd₁ and the graphical representation of the relative difference rd₂. Therefore, the calibration circuitry 110 extrapolates a value 483 between the relative difference rd₂ and the relative difference rd₃ in the illustrated embodiment. In the plot 480 of FIG. 4G, an extrapolation curve 482 is shown intersecting the peaks of the graphical representation of the relative difference rd₂ and the graphical representation of the relative difference rd₃. In this embodiment, the extrapolated value 483 corresponds to the point at which the extrapolation curve 482 crosses the x-axis of plot 480. The calibration circuitry 110 may, for example, determine a read level offset value that corresponds to the extrapolated value 483 (i.e., a voltage value dependent on the relative difference rd₂ corresponding to the base offset test voltage V_(B) voltage value and the relative difference rd₃ corresponding to the upper test signal V_(U1) voltage level. In another embodiment, the calibration circuitry 110 may determine a read level offset value corresponding to a distance between the base offset read level signal (e.g., the base offset test voltage V_(B)) and the extrapolated value 483. In another embodiment, the calibration circuitry 110 may determine a read level offset value corresponding to a distance between the upper test signal V_(U1) voltage level and the extrapolated value 483. In another embodiment, the calibration circuitry 110 may determine a read level offset value to be a test signal voltage that the extrapolated value 483 is closest to (e.g., the base offset test voltage V_(B) in the example shown in FIG. 4G). Other methods of determining the read level offset value utilizing offset read level test signals (including a base offset read level test signal that is offset from a current read level signal) may also be used in various embodiments.

In some embodiments, the read level offset value can be rounded to a nearest offset step value (e.g., the nearest 5 mV or 10 mV offset step value) to facilitate easier storage as an integer value (e.g. a byte and/or a signed integer value). For example, in the embodiment illustrated in FIG. 4G, where the base offset read level test signal V_(B) and the upper test signal V_(U1) are separated by 20 mV, the read level offset value can represent an offset of about 7 mV above the base offset read level test signal V_(B). If the memory device is configured to round read level offset values to the nearest ±10 mV offset step value, then the read level offset value of +7 mV would be rounded to a value of +10 mV (e.g., the calibration circuitry 110 would return a value of +10 mV indicating that the read level signal should be about 10 mV above the base offset read level test signal V_(B)). Similarly, if the read level offset value was calculated for another memory region to be 4 mV above the base offset read level test signal V_(B), and the memory device was configured to round a read level offset value to the nearest ±10 mV offset step value, then the read level offset value of +4 mV would be rounded to a value of 0 mV. In these embodiments, as discussed above, the read level offset actually used to update the current read level signal is adjusted based on the amount the base offset read level test signal V_(B) was offset from the current read level signal V_(A) by any of the calibration circuitry 110, the controller 106, and/or the host 108. Accordingly, even where the initial read level offset is, for example, rounded to a value of 0 mV as described above, the level at which the current read level signal is adjusted would be the amount the base offset read level test signal V_(B) was offset from the current read level signal V_(A). In another example, the level at which the current read level signal is adjusted when the read level offset value determined above is 10 mV would be 10 mV plus the amount the base offset read level test signal V_(B) was offset from the current read level signal V_(A).

In some embodiments, the memory region can be a memory page within a larger memory region (e.g., a memory block, a memory unit, etc.) and/or a memory block within a larger memory region (e.g., a memory unit). In these and other embodiments, read level calibration can be performed on more than one memory page and/or on more than one memory block within a larger memory region. As such, the calibration circuitry 110 can produce multiple read level offset values by performing read level calibration on the larger memory region (e.g., on all or a subset of the memory pages and/or on all or a subset of the memory blocks comprising the larger memory region). In such an embodiment, the calibration circuitry 110 can calculate an average read level offset value for the larger memory region from the multiple read level offset values in the manner described in greater detail with reference to FIGS. 5A-B, below.

FIGS. 5A and 5B are flow diagrams illustrating routine 570 and routine 580, respectively, for operating a memory device in accordance with an embodiment of the present technology. Routine 570 can be executed, for example, by the controller 106 (FIG. 1) of the memory device 100 (FIG. 1), and routine 580 can be executed, for example, by the calibration circuitry 110 (FIG. 1) of the memory component 102 (FIG. 1) of the memory device 100. In one embodiment, the routine 570 and the routine 580 can be carried out automatically after the controller 106 has programmed (e.g., written to and/or erased) one or more memory regions of the memory component 102 a predefined number of times (e.g., 1, 25, 100, 400, 800, 1000, 10000, etc. times). In other embodiments, the routine 570 and the routine 580 can be carried out in response to a calibration command that originates from the host device 108 (FIG. 1). In still other embodiments, the routine 570 and the routine 580 can be carried out upon the occurrence of other events, e.g., after a specified amount of time has elapsed, after the memory device 100 is first powered on or connected to a host device, when a memory device 100 is subjected to a particular temperature threshold, when a memory device is subjected to a particular threshold for a predetermined period of time, and/or upon completion of other commands (e.g., read commands).

Referring to FIG. 5A, the routine 570 begins determining a base offset read level (block 571). This determination can be predetermined (i.e., the level at which test signals will be offset is programmed into the host, controller, and/or memory device). In various embodiments, the level at which the read level test signals are offset may be determined by other factors, such as based on the occurrence of other events, e.g., after a specified amount of time has elapsed, after the memory device 100 is first powered on or connected to a host device, when a memory device 100 is subjected to a particular temperature threshold, when a memory device is subjected to a particular threshold for a predetermined period of time, and/or upon completion of other commands (e.g., read commands). In various embodiments, the level at which the read level test signals are offset may be determined by real time calculations by the host, controller, and/or memory device. For example, the amount to offset test signals for a calibration may be based on a magnitude of a previous read level offset value from a previous calibration. In another example, the amount to offset test signals for a calibration may be based on a determination by the host, controller, and/or memory device that a particular error condition(s) in reading a memory device has occurred. In various embodiments, other methods of determining a base offset read level at the block 571 may be utilized.

After determining the base offset read level, an offset calibration command is sent (block 572), such as the calibration command 232 of FIG. 2, to the routine 580 containing, e.g., one or more logical addresses of selected memory regions (e.g., of one or more selected memory pages, blocks, logic units, etc.) of the memory component 102 and/or one or more current read level signals (e.g., current read level voltages) of the selected memory regions. As discussed above, the current read level signals may be default read level signals (e.g., default read level voltages) initially programmed by the manufacturer and/or the current read level signals may be read level signals (e.g., read level voltages) previously calibrated in accordance with the present technology. In some embodiments, the one or more memory regions are indicated by the calibration command 572 (e.g., the regions can be selected by the controller 106 or the host device 108). In other embodiments, the calibration circuitry 110 can select the one or more memory regions in response to the calibration command 572 and/or retrieve (e.g., from a table stored within the memory component 102 and/or the embedded memory 132 of controller 106) the corresponding current read level signal(s) for the selected memory region(s).

Referring now to FIG. 5B, the routine 580 receives the offset calibration command (block 581). The routine 580 proceeds to perform the offset read level calibration on the selected memory regions to produce one or more read level offset values (block 582) in accordance with the discussion of FIGS. 4A-G above. For example, the selected memory regions can be one or more memory units, and the routine 580 can perform offset read level calibration on a predefined number of memory blocks (e.g., a sampled subset of memory blocks) of each memory unit by, for example, performing read level calibration on a predefined number of memory pages per word line group (e.g., two edge memory pages and a middle memory page per word line group) of each memory block. In other embodiments, the selected memory regions can be one or more memory blocks, and the routine 580 can perform offset read level calibration on a predefined number of memory pages per word line group (e.g., a sampled subset of memory pages per word line group) within the selected memory blocks.

After the routine 580 obtains the one or more read level offset values produced by performing offset read level calibration on the selected memory region(s) (block 582), the routine 580 can calculate an average read level offset value (e.g., per memory page, per word line group, per memory block, per memory unit, etc.) (block 583) from the obtained read level offset value(s). For example, the routine 580 can calculate an average read level offset value by taking the median of the obtained read level offset values (e.g., a median byte and/or a median signed integer value). In other embodiments, the routine 580 can calculate the average read level offset value using other averaging techniques (e.g., mean, mode, etc.). In these and other embodiments, the routine 580 can omit outlier read level offset values (e.g., values greater than ±10 digital to analog (DAC) offsets) obtained from performing offset read level calibration on the selected memory region(s) (block 582) before calculating the average read level offset value (block 583). In some embodiments, the routine 580 can calculate the average read level offset value before rounding to the nearest offset step value. In other embodiments, the routine 580 can calculate the average read level offset value after rounding the obtained read level offset values to the nearest offset step value. Furthermore, in embodiments that produce a single read level offset value after performing offset read level calibration on the selected memory region(s), the single read level offset value can be treated as an average read level offset value for the selected memory region(s). The routine 580 can then output the average read level offset value to the routine 570 (block 584).

Referring again to FIG. 5A, the routine 570 can receive the average read level offset value from the routine 580 (block 573). The routine 570 can then use the average read level offset value to update the current read level signal for the selected memory region (block 574). As discussed above, if the read level offset value does not incorporate the level at which the calibration test signals were offset from the current read level signals, that level may be incorporated before and/or when updating the current read level signal. The routine 570 can use the average read level offset value (e.g., when the average read level offset value is represented as a byte and/or as a signed integer value) to update a stored calibration value for the current read level signal of the selected memory region (block 584). In these embodiments, the current read level signal for the selected memory region can be represented as, for example, the default read level signal plus a calibration value plus any other system offsets. The calibration value can initially be set equal to zero when the memory device 100 is initially configured (e.g., at the time of manufacture, or upon initialization), and the routine 570 can update the calibration value by, e.g., adding the average read level offset value received from the routine 580 to the previous calibration value to obtain a new calibration value. For example, in one embodiment, if the previous calibration value was +5 mV (e.g., representing a 5 mV offset above an original read level signal), and the average read level offset value determined by a read level calibration operation is +10 mV (e.g., representing a 10 mV offset above a center test signal of the read level calibration), then the routine 580 can update the stored calibration value by summing the stored calibration value with the average read level offset value to obtain an updated calibration value (e.g., +5 mV summed with +10 mV returns +15 mV).

Accordingly, in subsequent iterations of read level calibration of the selected memory region (either offset calibrations or non-offset calibrations), the calibration value can be updated by adding the newly-obtained average read level offset value(s) from the routine 580 to the stored calibration value. Thus, the updated calibration value(s) can represent instructions to increase or decrease the current read level signal for the memory region (e.g., by ±5 mV, ±10 mV, ±20 mV, and/or other voltage values) relative to a previous read level signal for the selected memory region in order to arrive at an improved read level signal for the memory region. In these and other embodiments, the routine 570 can store the updated calibration value and/or the updated current read level signal in, for example, a table stored on the memory component 102 and/or embedded memory 132 of the controller 106 of the memory device 100, so that routine 570 can continue to track these values (e.g., as persistent data to be loaded upon each power up of the memory device 100).

While in the foregoing exemplary embodiments, offset read level calibration operations have been described as outputting values representing offsets by which read level signals can be indexed, the present technology is not limited to this arrangement. In other embodiments, the routine 570 can translate the average read level offset value into, e.g., a scalar value that can represent a scale factor to apply to the current read level signal for the memory region in order to arrive at an improved read level signal. In still other embodiments, a read level calibration operation can translate a calculated average read level offset value into other instructions to update or otherwise modify the current read level signal (e.g., as a read level signal value, instead of an offset to a stored value, etc.).

In accordance with another aspect of the present disclosure, the foregoing offset read level calibration operations can be performed during an error recovery operation, providing an increased likelihood of success in recovering from, e.g., a read error caused by the undesired shift of a threshold voltage. In this regard, error recovery operations can involve a variety of remedial operations, which may be performed in a predetermined sequence (e.g., in an order sorted by increasing energy consumption, delay, or other cost function). One such sequence can involve multiple read retry operations with adjusted threshold voltages (e.g., alternating between increased and decreased threshold voltages). Read retry operations are generally low cost (e.g., in terms of energy and delay) to implement, but may not be successful for errors caused by larger threshold voltage shifts. Accordingly, if read retry operations do not resolve the error, one or more offset read level calibration operations such as those set forth above can be tried (e.g., with offsets corresponding to the largest offsets attempted by the read retry operations). Including one or more such offset read level calibration operations in an error recovery flow can provide not only an increased likelihood of resolving an error, but can further provide information corresponding to an updated threshold voltage level, as set forth in greater detail above (e.g., reducing the likelihood of future errors at the same location due to the large threshold voltage shift).

FIG. 6 is a schematic view of a system that includes a memory device in accordance with embodiments of the present technology. Any one of the foregoing memory devices described above with reference to FIGS. 1-5B can be incorporated into any of a myriad of larger and/or more complex systems, a representative example of which is system 690 shown schematically in FIG. 6. The system 690 can include a semiconductor device assembly 600, a power source 692, a driver 694, a processor 696, and/or other subsystems and components 698. The semiconductor device assembly 600 can include features generally similar to those of the memory device described above with reference to FIGS. 1-5B, and can, therefore, include various features that calibrate read level signals. The resulting system 690 can perform any of a wide variety of functions, such as memory storage, data processing, and/or other suitable functions. Accordingly, representative systems 690 can include, without limitation, hand-held devices (e.g., mobile phones, tablets, digital readers, and digital audio players), computers, vehicles, appliances, and other products. Components of the system 690 may be housed in a single unit or distributed over multiple, interconnected units (e.g., through a communications network). The components of the system 690 can also include remote devices and any of a wide variety of computer readable media.

From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. For example, in one embodiment, the calibration circuitry 110 (FIG. 1) can select one or more memory regions to be calibrated absent a calibration command (e.g., a calibration command 232 from the controller 106 and/or from the host device 108; FIGS. 1-2). The calibration circuitry 110 can select the memory regions to be calibrated, for example, based on factors such as the number of times a memory region has been read, erased, and/or written to; the amount of time that has elapsed since the memory region was last calibrated; random sampling (e.g., of one or more memory pages within a memory block, of one or more memory blocks within a memory unit, etc.); and/or in accordance with a predefined order of memory regions. In these and other embodiments, a single component (e.g., the calibration circuitry 110) of system 101 (FIG. 1) can perform each of the steps of the routine 570 and the routine 580 discussed above with respect to FIGS. 5A-B (e.g., absent a calibration command). In addition, certain aspects of the new technology described in the context of particular embodiments may also be combined or eliminated in other embodiments. Moreover, although advantages associated with certain embodiments of the new technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described. 

Me claim:
 1. A system comprising: a memory region comprising a plurality of memory cells; and calibration circuitry operably coupled to the memory region configured to: apply a plurality of read level test signals to a subset of the plurality of cells, determine, for each applied read level test signal of the plurality, a count of the subset that outputs a predetermined data state, compare the counts corresponding to adjacent ones of the plurality of real level test signals to determine a plurality of count differences, compare a minimum count difference of the plurality of count differences to each other count difference of the plurality to determine a plurality of relative count differences, each relative count difference corresponding to one of the plurality of read level test signals; output a read level offset value based on determining the plurality of relative count differences.
 2. The system of claim 1, wherein the calibration circuitry is further configured to calculate the read level offset value based on comparing a positive relative count difference of plurality to an adjacent negative relative count difference of the plurality.
 3. The system of claim 1, wherein the calibration circuitry is further configured to calculate the read level offset value based on a relative count difference with a smallest absolute value of the plurality of relative count differences.
 4. The system of claim 1, wherein the memory region comprises a memory block including a plurality of word line groups, and wherein the subset comprises a portion of the plurality of word line groups.
 5. The system of claim 1, wherein the memory region comprises a memory block including a plurality of word line groups, each word line group having a plurality of memory pages, and wherein the subset comprises two endmost memory pages and an inner memory page from each word line group.
 6. The system of claim 1 wherein the calibration circuitry is configured to output the read level offset value in response to receiving a calibration command.
 7. The system of claim 1, wherein the calibration circuitry is located on a same memory die as the memory region.
 8. The system of claim 1, wherein the plurality of read level test signals include a read level signal of the memory region, at least two signals offset above the read level signal, and at least two signals offset below the read level signal.
 9. A method for calibrating a memory region of a system, the method comprising: applying a plurality of read level test signals to a subset of memory cells of the memory region, determining, for each applied read level test signal of the plurality, a count of the subset that outputs a predetermined data state, comparing the counts corresponding to adjacent ones of the plurality of real level test signals to determine a plurality of count differences, comparing a minimum count difference of the plurality of count differences to each other count difference of the plurality to determine a plurality of relative count differences, each relative count difference corresponding to one of the plurality of read level test signals; outputting a read level offset value based on determining the plurality of relative count differences.
 10. The method of claim 9 wherein the plurality of read level test signals include a read level signal of the memory region, at least two signals offset above the read level signal, and at least two signals offset below the read level signal.
 11. The method of claim 9, further comprising calculating the read level offset value based on comparing a positive relative count difference of plurality to an adjacent negative relative count difference of the plurality.
 12. The method of claim 9, further comprising calculating the read level offset value based on a relative count difference with a smallest absolute value of the plurality of relative count differences
 13. The method of claim 9 wherein the memory region comprises a memory block including a plurality of word line groups, and wherein the subset comprises a portion of the plurality of word line groups.
 14. The method of claim 9 wherein the memory region comprises a memory block including a plurality of word line groups, each word line group having a plurality of memory pages, and wherein the subset comprises two endmost memory pages and a middle memory page from each word line group.
 15. A non-transitory computer readable medium having instructions stored thereon that, upon execution by a computing device, cause the computing device to perform operations, wherein the instructions comprise: instructions to apply a plurality of read level test signals to a subset of a plurality of cells of a memory region, instructions to determine, for each applied read level test signal of the plurality, a count of the subset that outputs a predetermined data state, instructions to compare the counts corresponding to adjacent ones of the plurality of real level test signals to determine a plurality of count differences, instructions to compare a minimum count difference of the plurality of count differences to each other count difference of the plurality to determine a plurality of relative count differences, each relative count difference corresponding to one of the plurality of read level test signals; instructions to output a read level offset value based on determining the plurality of relative count differences.
 16. The non-transitory computer readable medium of claim 15, wherein the instructions further comprise instructions to calculate the read level offset value based on comparing a positive relative count difference of plurality to an adjacent negative relative count difference of the plurality.
 17. The non-transitory computer readable medium of claim 15, wherein the instructions further comprise instructions to calculate the read level offset value based on a relative count difference with a smallest absolute value of the plurality of relative count differences.
 18. The non-transitory computer readable medium of claim 15, wherein the plurality of read level test signals include a read level signal of the memory region, at least two signals offset above the read level signal, and at least two signals offset below the read level signal. 